Einstein's Theory of General Relativity, which describes how gravity works, turns 100 this month. The theory has successfully explained a lot of what we observe out in the universe; but there are signs that it's incomplete. In the 1990s, astronomers observed that the expansion of the universe is speeding up, as if some mysterious force is pushing everything apart faster and faster. Nearly 20 years later, one of the biggest unanswered questions in science is: what is this dark energy? Not only was dark energy not predicted by general relativity, but its mere existence might mean that the theory needs to be tweaked or even replaced.

When Richard Finnell first met her, Rachel was a nine-year-old girl with severe developmental delays. Her condition seemed to be caused by a deficiency in a critical B vitamin called folate. Yet she had plenty of folate circulating in her blood. Somehow it was vanishing before it got to her brain and spine. Eventually Finnell made a surprising discovery. He's now using new genetic tools like CRISPR to better understand her condition and test possible therapies.

How do we manage to follow a conversation with a friend in the middle of a noisy room? Neuroscientists, like Nace Golding, are still working out the details—but what they've learned so far is pretty amazing.